1. Field of the Invention
The present invention relates to an analog resistive-film touch panel.
2. Description of the Related Art
A touch panel is an input device typically used to provide input directly on a display screen, and is implemented by combining a coordinate detection function with a display function. Unlike other pointing devices such as a mouse, etc., that provide relative coordinate input, a touch panel, which enables an operator to easily input absolute coordinates, and can provide an interface that better matches the characteristics of human perception. Touch panels are classified as a capacitive-type, electromagnetic inductive-type, ultrasonic-type, etc., according to how the pressing on the panel is sensed. A typical one is the analog resistive-film type described, for example, in Japanese Unexamined Patent Publication No. 2006-39667. An analog resistive-film touch panel comprises planar sheet-like first and second members whose facing surfaces are each coated with a transparent conductive film, and when one surface where a potential gradient is formed across the transparent conductive film is caused to touch the transparent conductive film on the other surface by a touch operation on the panel, the absolute coordinates of the touched point on the panel are determined based on the voltage value detected at that other surface.
FIG. 20 is a perspective view illustrating a conventional analog resistive-film touch panel, and FIG. 21 is a cross-sectional view of the conventional analog resistive-film touch panel. It is to be understood that, throughout the different drawings given herein, the same reference numerals designate component elements having the same functions.
Generally, an analog resistive-film touch panel 100 comprises a film 111 and a glass 112, whose facing surfaces are each coated with a transparent conductive film 114 formed from ITO (indium tin oxide), which is a transparent conductive substance. The surface of the film 111 opposite from the surface thereof facing the glass 112 provides the touch operation surface of the touch panel 100. The film 111 and the glass 112 are bonded together, for example, by a double sided adhesive tape. The touch panel 100 is electrically connected to a host (not shown) by a cable 113.
FIGS. 22(a) and 22(b) are diagrams explaining the coordinate detection principle commonly employed in the analog resistive-film touch panel. As shown in FIG. 22(a), an electrode pair consisting of electrodes 121-1 and 121-2 is formed on the ITO-coated surface of the film 111 of the touch panel 100, and an electrode pair consisting of electrodes 122-1 and 122-2 is formed on the ITO-coated surface of the glass 112. The electrodes 121-1 and 121-2 are arranged so as to face each other in a direction orthogonal to the direction in which the electrodes 122-1 and 122-2 face each other.
When the film 111 of the touch panel 100 is touched by a finger, the film 111 deflects in substantially the vertical direction, causing the transparent conductive film formed on the film 111 to touch the transparent conductive film formed on the glass 112 (see point A in the figure). In the illustrated example, when a drive voltage of, for example, 5 volts is applied between the electrodes 121-1 and 121-2, a voltage value Va is detected, as shown in FIG. 22(b), via the electrodes 122-1 and 122-2 formed on the surface of the glass 112. The absolute coordinate of the touched point on the touch operation surface along the direction in which the electrodes 121-1 and 121-2 face each other can be calculated from the ratio of the detected voltage value Va to the voltage value of the voltage (i.e., 5 volts) applied between the electrodes 121-1 and 121-2. Since the method of calculation using such a ratio is based on the premise that the potential gradient of the same potential formed by the drive voltage applied between the electrodes 121-1 and 121-2 changes linearly, the input area where the touch panel 100 can be touched for operation is more or less limited.
FIGS. 23(a) and 23(b) are diagrams explaining the problem that may arise when the analog resistive-film touch panel illustrated in FIGS. 22(a) and 22(b) is touched by a plurality of fingers for operation. As shown in FIG. 23(a), when the film 111 of the touch panel 100 is touched by a plurality of fingers for operation (two fingers in the illustrated example), the film 111 deflects in substantially the vertical direction, causing the transparent conductive film formed on the film 111 to touch the transparent conductive film on the glass 112 at two points (points A and B in the figure). As a result, a voltage value Vc intermediate between the voltages at points A and B is detected, as shown in FIG. 23(b), via the electrodes 122-1 and 122-2 formed on the surface of the glass 112. This means that, when the analog resistive-film touch panel is touched by a plurality of fingers for operation (the so-called multi-touch operation), the plurality of pressed points on the touch panel surface cannot be recognized.
A capacitive type touch panel that can handle such a multi-touch operation is already developed. However, unlike the analog resistive-film type, the capacitive type, because of its operating principles, cannot accept pen input. Furthermore, the capacitive type is not suitable for use in an environment susceptible to static electricity or moisture.
In view of the above problem, it is an object of the present invention to provide an analog resistive-film touch panel that can recognize and discriminate a plurality of pressed positions in a multi-touch operation.